George David Banks, the former White House adviser on energy and climate, at United Nations climate talks in Bonn, Germany, in November.CreditLukas Schulze/Getty Images

The White House’s senior adviser on energy and climate change stepped down last week after being denied a full security clearance for smoking marijuana about five years ago. Now he is speaking his mind about the Paris climate agreement that President Trump has disavowed.

“I’m going to say something controversial,” the former adviser, George David Banks, said in an interview. “The Paris agreement is a good Republican agreement. It’s everything the Bush administration wanted.”

Mr. Banks stood by Mr. Trump’s decision last summer to withdraw the United States from the global accord, despite having urged him behind the scenes to remain in. Since then, a large part of his work as the White House adviser on international energy issues has been to quietly seek ways that the United States might rejoin the deal.

That came to an abrupt end last week when Mr. Banks resigned after being told he would not be granted a full security clearance. In a wide-ranging interview days after leaving his position, Mr. Banks spoke about the surprise of being told that the three or four times he had smoked marijuana between 2009 and 2013, which he had self-reported to the F.B.I. in April 2017, were the reason for the security rejection.

He also spoke about his efforts to persuade Mr. Trump to remain open-minded about the Paris agreement, and the likelihood of the United States formally rejoining the accord during Mr. Trump’s presidency.

“A lot can happen between now and 2020,” Mr. Banks said, referring to the date when countries can formally leave the Paris agreement. He pointed to the president’s recent comment, which he made during a news conference with Prime Minister Erna Solberg of Norway, that “we could conceivably go back in” to the deal.

When Mr. Banks joined the White House in February 2017 to serve on both the National Security Council and National Economic Council, the fight was just heating up over whether to remain in the accord or “cancel” it, as Mr. Trump had promised to do on the campaign trail.

As on other issues, including trade and immigration, the Trump White House was divided between moderates and hard-liners. Gary D. Cohn, Mr. Trump’s chief economic adviser, Secretary of State Rex W. Tillerson and the president’s daughter Ivanka Trump and her husband, Jared Kushner, favored upholding the agreement. The charge to withdraw was led by the Environmental Protection Agency’s administrator, Scott Pruitt, and by Steve Bannon, who was Mr. Trump’s chief strategist at the time.

Mr. Banks said he seized on other comments Mr. Trump had made during the campaign — that he would “renegotiate” a Paris deal — hoping there might be an opening to persuade the president to stay in. He made the case then, and continues to now, that the United States could simply weaken the target that former President Barack Obama had pledged — to cut domestic greenhouse gas emissions at least 26 percent below 2005 levels by 2025 — while still remaining a member.

Mr. Trump ultimately rejected that argument, and declared in a ceremony in the Rose Garden last year that the Paris agreement “disadvantages the United States to the exclusive benefit of other countries.” The United States, he said, would immediately stop working toward its emissions target and would stop funding efforts to help developing countries tackle climate change. Mr. Trump has said several times since that he might reconsider his position if new, unspecified terms are made available.

Previously the executive vice president of the American Council for Capital Formation, a business-oriented think tank, Mr. Banks began his career as an economic analyst at the C.I.A. He served in the State Department and the White House Council on Environmental Quality, both under former President George W. Bush.

In the aftermath of the 1997 Kyoto Protocol, the first United Nations-led climate agreement (which the United States signed but did not ratify), he had a hand in helping the Bush administration outline what a climate deal would need to look like to win American support. It was an effort, he said, that would help shape his outlook on the Paris deal.

The Senate in 1997 unanimously rejected the Kyoto Protocol because it demanded legally binding emissions cuts from industrialized countries while allowing developing nations to act voluntarily. For any future deal to be acceptable, Congress instructed, it would need to hold all countries equally legally accountable for cutting greenhouse gas emissions. Emissions goals offered by nations would need to be voluntary and “bottom up” — that is, not dictated by the United Nations but rather devised domestically so they could be enforceable at home.

Those are essentially the parameters of the Paris agreement that the Obama administration negotiated with nearly 200 nations and finalized in 2015. “It’s a climate policy based on U.S. national interest that the Bush team started and the Obama team kept,” Mr. Banks said.

He called Mr. Obama’s emissions pledge “awful,” describing it as unrealistically high. He also criticized the Obama administration for failing to consult with Congress or industry in devising it. But, he said, “I think probably the United States got the best deal it could as a framework.”

Democrats and others who shaped the Paris agreement under the Obama administration argued the target was the same one the House of Representatives approved as part of climate legislation in 2009. As for the terms of the accord, they agreed with Mr. Banks, up to a point. The Obama administration was able to convince other countries to embrace the Bush-era terms, they said, precisely because it put forward an ambitious target that showed the world that America, the biggest emitter of greenhouse gases in history, was serious about tackling climate change.

“Paris fixes the problems Republicans had with the Kyoto Protocol — all countries are in, and each country picks its own target,” said Susan Biniaz, a former State Department legal adviser who worked on climate negotiations under both parties beginning in 1989. “On top of that, the targets aren’t legally binding. It’s what we who worked for the Bush administration tried to get, but it probably took a Democratic administration with more climate credibility to achieve it.”

“The Paris agreement was flawed from the beginning — a shortsighted approach that set unattainable targets for the United States while not holding international partners to the same standards,” said Senator James Inhofe, Republican of Oklahoma, who has called climate change a hoax. “President Trump was right to withdraw from it.”

The White House did not respond to a request to comment on Mr. Banks’s views.

Mr. Banks acknowledged that his was the minority position in an administration that ultimately was eager to keep Mr. Trump’s campaign promise and to undo Mr. Obama’s broader climate change agenda. Still, he insisted that Mr. Trump remains open to returning to the Paris agreement.

Mr. Banks argues that it would be in the interest of the United States, and even of fossil fuel industries, for America to rejoin the international discussion about curbing greenhouse gases. “The climate agenda is not going to go away any time soon, and if you’re not engaged aggressively, actively, there are going to be policies that are detrimental to the United States,” he said.

Lisa Friedman reports on climate and environmental policy in Washington. A former editor at Climatewire, she has covered nine international climate talks. @LFFriedman